With the reduction of CO2 emission recently required to prevent global warming, fuel efficiency has been improved in automobile engines. For this purpose, improvement has been made to reduce the friction of sliding portions in a piston system.
Tension reduction is important in piston rings, particularly in oil rings whose tension occupies 50% or more of the entire tension of piston rings. To this end, the tolerance range reduction of the tension is also required.
However, the improvement of combustion efficiency by the elevation of combustion temperatures or by the direct injection of fuel causes the problems that oil sludge generated due to the degradation of engine-lubricating oils is stuck to side rails and spacer expanders, and that side rails and seating tabs of spacer expanders coming into contact with inner peripheral surfaces of the side rails are worn.
The tension reduction of oil rings results in decrease in conformability to cylinder wall surfaces, drastically increasing oil consumption, and the sticking and wearing of oil rings.
With respect to conformability, a method of increasing a conformability coefficient by reducing the widths of side rails has been proposed. With respect to the sticking of oil rings, a method of forming an oil-repellent film such as a fluorine-containing film on the oil ring surface has been proposed. With respect to wearing, a method of plating chromium on oil rings, and a method of nitriding oil rings have been proposed.
With respect to the width reduction of a piston ring, Patent Reference 1 specifically discloses a compression ring as wide as 1.0 mm or less, and a combined oil control ring as wide as 2.0 mm or less.
With respect to the sticking of an oil ring, Patent Reference 2 discloses a method for forming an oil-repellent coating by a sol-gel method from a metal alkoxide and fluoroalkyl-substituted metal alkoxide, in which part of alkoxy groups are substituted by fluoroalkyl groups.
With respect to wearing, Patent Reference 3 discloses the nitriding of a spacer expander, and WO 2005/040645 discloses the formation of a gas-nitrided layer as thick as 10-60 μm, which contains special S phases having peaks at 2θ=40° and 46°, respectively, in X-ray diffraction with Cu-Kα, as a nitrided layer having excellent corrosion resistance.
However, increase in the area and thickness of a nitrided layer leads to increase in the Young's modulus and developed length (circumferential length) of a spacer expander, resulting in larger variation of tension, and thus making it difficult to nitride a spacer expander within a predetermined accuracy range. For example, when a nitrided layer as thick as 30 μm is formed by gas-phase nitriding on an entire surface of the spacer expander, the tension increases 18 N than before forming the nitrided layer, resulting in difficulty in controlling the tension.
To reduce the variation of tension, Patent Reference 5 teaches a method for producing a spacer expander comprising a first step of forming a nitridation-preventing layer of Ni, Cr or Cu as thick as 1-5 μm on a surface of a wire for a spacer expander, a second step of providing the wire with a corrugated shape by using gears, a third step of forming seating tabs by shearing on an inner peripheral surface of the corrugated wire, and a fourth step of nitriding the sheared surfaces. The base material of a spacer expander body, particularly its corrugated corner portions, is exposed by gear rolling, in case that Ni plating layers on the corrugated corner portions are cracked, peeled, or made thin. To prevent the base material from being exposed, Patent Reference 6 teaches that other surfaces than the sheared surfaces should be covered with a Ni plating film as thick as 1-7 μm having a Ni-diffused layer.
Patent Reference 7 teaches that the Ni plating film suppresses the sticking of oil sludge, because of its surface free energy and hydrogen bonding force.
As described above, it has been confirmed that a Ni plating film well functions as a nitridation-preventing film or an anti-sticking film. However, the Ni plating film per se may be chipped or peeled depending on the production conditions, practically needing further investigation of the preferred structure of a Ni plating film.